Ion exchange resins have been in use for many years for the purpose of softening hard water. Their utility derives from their tendency to adsorb certain ions in preference to others. In particular, in softening systems they will remove undesirable cations such as calcium or magnesium from hard water, substituting in their place more benign cations such as sodium.
Once all adsorption sites on the resin have been filled with hard water ions the system needs to be regenerated. This is done by exposing the resin to a highly concentrated solution of sodium. Following the law of mass action, the adsorption reaction is now driven in the reverse direction. The sites are once more filled with sodium ions and the `bad` cations are driven out into solution once more where they can be readily disposed of.
With the advent of the semiconductor industry, a need for large amounts of deionized (DI) water arose. In this instance, the most common cation in need of removal was sodium while the most common anion was chlorine. Accordingly, DI water generation units of the kind illustrated in FIG. 1 were developed. Column shaped container 10 is packed with cation exchange resin (CER) 12 and anion exchange resin (AER) 14. Water in need of de-ionization 11 enters through inlet tube 17 and is directed to the top of the column. It then trickles down through CER 12 and AER 14 before emerging as DI water 13.
In the CER the sodium ions are exchanged for hydrogen ions while in the AER the chloride ions are exchanged for hydroxyl ions. As they are released, the hydrogen and hydroxyl ions recombine to form water molecules so the net number of ions in the water is reduced.
In production line situations, where any given water de-ionizing unit is operating continuously, it becomes necessary to regenerate the CER and AER on a daily, weekly, or monthly basis, depending on the tank size and the flow rate. As in the water softener case, regeneration is forced by immersing the resins in high concentration solutions of the `good` ions. In this instance, hydrochloric acid for the CER and sodium hydroxide for the AER.
Because costs need to be controlled very closely, it is important that regeneration take place exactly when it is needed. If regeneration takes place too soon, the deionizing unit is out of action sooner than necessary and more regenerating chemicals will be consumed than necessary. If it takes place too late, the unit stops producing deionized water, with disastrous consequences for the production line.
Since the rate at which DI water emerges from the unit is governed by the demands of the line and is therefore constantly changing, simply measuring the time since the previous regeneration is an inadequate solution to this problem. Nor is monitoring the ion concentration of the outputted water an adequate solution since, by the time an increase in ion concentration has been detected, it is too late. Instead, it is necessary to keep track of the total amount of impurity ions that the resins have adsorbed so that when the accumulated dosage equals the known capacity of the system, recharging of the resins can take place on a `just in time` basis.
A dosimeter based on the above approach has been described by O'Brien et al. (U.S. Pat. No. 3,676,336 Jul 1972). The quantity of liquid that is outputted is monitored by means of a flowmeter. The device used by O'Brien generated an electrical signal that was proportional to the square of the flow rate so a square root extractor was provided to convert to a linear signal.
The quality of the liquid inputted by O'Brien et al.'s device was continually monitored by the emission of a signal that was proportional to the electrical conductivity of the untreated DI water. The two signals were then multiplied together and, if their product exceeded some preset value, a pulse was sent to a counter. When the pulse count exceeded some preset value the unit signalled a `time to regenerate` message.
While the O'Brien unit was an improvement over what had previously been available, it suffered from several deficiencies. In particular, conductivity of the water is only an approximate indication of ion concentration and counting only those flow.times.conductivity products that exceed some threshold value can also lead to significant error. These limitations of O'Brien et al. will be discussed in greater detail below.
Other prior art that we found to be of interest included Le Dall (U.S. Pat. No. 4,275,448 Jun 1981) and Seal (U.S. Pat. No. 4,490,249 Dec 1984). Le Dall's system monitors and also controls the total volume of liquid that has passed through it. The hardness of the incoming liquid is used as a guide to determining what this volume should be. The quality of the outputted (that is, softened) liquid is monitored by measuring its conductivity. Seal uses a hardness meter and a flowmeter to drive a microcomputer. The latter keeps track of the amount of unsoftened water that has flowed through the system and uses this information, along with the known hardness of the water and the capacity of the system, to determine when it is time to regenerate.